What happened
On October 20, 2012, a JAL Express Boeing 737-800, registration JA342J, was performing a scheduled flight from Tokyo International Airport to Matsuyama Airport. The aircraft was carrying 138 passengers and 6 crew members. During the climb to approximately 13,000 feet, the flight crew observed a decrease in the rotating speed of the No. 1 engine accompanied by an increase in exhaust gas temperature. Following standard procedures, the crew shut down the engine, and the aircraft returned to Tokyo International Airport for an emergency landing.
The investigation
The Japan Transport Safety Board (JTSB) examined the engine to determine the cause of the internal damage. Investigators discovered significant damage within the High Pressure Compressor (HPC). Specifically, all blades in the fifth stage of the HPC had fractured from their roots, and the fourth stage blades showed dents on their trailing edges. Additionally, the combustion chamber and turbines exhibited damage, including burnout traces.
Analysis of the fifth stage blades revealed evidence of fatigue crack propagation caused by repeated loading. Investigators also identified a thick layer of material, described as a "scab," at the bottom of the fifth stage HPC casing. This material was found to be the same as the blade material, indicating intense contact between the blade tips and the casing. Furthermore, traces of dried liquid, identified as likely rainwater or seawater, were found accumulated in cavities at the bottom of the HPC casing.
Findings
The primary cause of the engine damage was the contact between the fifth stage HPC blade tips and the engine casing, which applied heavy loads to the blade roots, initiating cracks that eventually led to the separation of all fifth-stage blade airfoils.
This contact was likely facilitated by a reduction in the clearance between the blades and the casing. The investigation suggests that the accumulation of water in the lower cavities of the HPC casing may have locally suppressed the thermal expansion of the casing during engine start, causing the casing to deform and narrow the gap. The relatively new status of the engine, having been in service for only about seven months, may have also contributed to tighter-than-usual clearances.